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Software ==>	SQL (129 exercises)
Topic ==>	Derived tables and CTEs (13 exercises)
Level ==>	Relatively easy
Subject ==>	SQL training

Use a Common Table Expression to write a complex query in two distinct parts

This exercise is provided to allow potential course delegates to choose the correct Wise Owl Microsoft training course, and may not be reproduced in whole or in part in any format without the prior written consent of Wise Owl.

Before you can do this exercise, you'll need to download and unzip this file.

First (if you haven't already done so) run the script shown above to generate the Doctor Who training database.

The aim of this exercise is to use a CTE (Common Table Expression) to extract data from the database in two passes:

Get a list of all of the episodes written by authors with MP in their names.
Get a list of the companions featuring in these episodes.

You get no points for pointing out that you could do all of this in a single query!

Start by creating a query to show just the episode id numbers for those Doctor Who episodes written by authors with MP as part of their names:

This should return 4 episodes (one written by Keith Temple, as it happens, and 3 by Steve Thompson).

Now create a CTE from this, using the standard syntax:

-- previous command must end with a semi-colon (;)

WITH NameForYourCte AS (

SELECT ...

)

-- now use this in a query

SELECT

...

FROM

NameForYourCte

-- join this to other tables as normal

INNER JOIN ...

Your final query should join to your CTE to show the companions featuring in these episodes (you may need to use the word DISTINCT to avoid some companions' names appearing twice):

The results: 4 companions feature in episodes written by authors whose names contain MP.

Optionally, save your query as If that is not useful.sql, then close it down.

You can unzip this file to see the answers to this exercise, although please remember this is for your personal use only.

You can find other training resources for the subject of this exercise here:

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EXERCISE TOPIC

SQL EXERCISES

SQL EXERCISES

Use a Common Table Expression to write a complex query in two distinct parts

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